Sputtering is known in the art as a technique for depositing layers or coatings onto substrates. For example, a low-emissivity (low-E) coating can be deposited onto a glass substrate by successively sputter-depositing a plurality of different layers onto the substrate. As an example, a low-E coating may include the following layers in this order: glass substrate/SnO2/ZnO/Ag/ZnO, where the Ag layer is an IR reflecting layer and the metal oxide layers are dielectric layers. In this example, one or more tin (Sn) targets may be used to sputter-deposit the base layer of SnO2, one or more zinc (Zn) inclusive targets may be used to sputter-deposit the next layer of ZnO, an Ag target may be used to sputter-deposit the Ag layer, and so forth. The sputtering of each target is performed in a chamber housing a gaseous atmosphere (e.g., a mixture of Ar and O gases in the Sn and/or Zn target atmosphere(s)). In each sputtering chamber, sputtering gas discharge is maintained at a partial pressure less than atmospheric.
Example references discussing sputtering and devices used therefore include U.S. Pat. Nos. 5,427,665, 5,725,746 and 2004/0163943, the entire disclosures of which are all hereby incorporated herein by reference.
A sputtering target (e.g., cylindrical rotatable magnetron sputtering target) typically includes a cathode tube within which is a magnet array. The cathode tube is often made of stainless steel. The target material is formed on the tube by spraying, casting or pressing it onto the outer surface of the stainless steel cathode tube. Often, a bonding or backing layer is provided between the tube and the target to improve bonding of the target material to the tube. Each sputtering chamber includes one or more targets, and thus includes one or more of these cathode tubes. The cathode tube(s) may be held at a negative potential (e.g., −200 to −1500 V), and may be sputtered when rotating. When a target is rotating, ions from the sputtering gas discharge are accelerated into the target and dislodge, or sputter off, atoms of the target material. These atoms, in turn, together with the gas form the appropriate compound (e.g., tin oxide) that is directed to the substrate in order to form a thin film or layer of the same on the substrate.
There are different types of sputtering targets, such as planar magnetron and cylindrical rotatable magnetron targets. Planar magnetrons may have an array of magnets arranged in the form of a closed loop and mounted in a fixed position behind the target. A magnetic field in the formed of a closed loop is thus formed in front of the target. This field causes electrons from the discharge to be trapped in the field and travel in a pattern which creates a more intense ionization and higher sputtering rate. Since sputter is mainly performed in the zone defined by the magnetic field, a racetrack shaped erosion zone is produced as sputtering occurs. In other words, the target material is unevenly sputtered off of the target during sputtering in such planar magnetron targets.
Rotating magnetron targets, including the tube and target material, were developed to overcome erosion problems of planar magnetrons. An example conventional rotating magnetron target 10 is shown in FIG. 5, in cross section (see also U.S. Pat. No. 6,787,003, the disclosure of which is hereby incorporated herein by reference). The magnetron target 10 shown in FIG. 5 includes cathode tube 20 which may be made of stainless steel or the like, target material 30 provided on the cathode tube, and relatively thin bonding layer 40 provided on the cathode tube between and contacting the cathode tube 20 and the target material 30. The bonding layer 40 is applied at a uniform thickness along the length of tube 20, and helps insure that the target material 30 is securely adhered to the cathode tube 20. The bonding layer 40 is typically conductive and may have a coefficient of thermal expansion between that of the hollow tube 20 and the target material 30. An example material for bonding layer40 is nickel mixed with aluminum. The target material 30 and bonding layer 40 are typically applied to the tube 20 via plasma spraying or the like.
In the case of rotating magnetrons such as that shown in cross section in FIG. 5, the cathode tube 20, bonding layer 40, and target material 30 thereon are rotated over a magnetic array (that is often stationary) that defines a sputtering zone. Due to the rotation, different portions of the target are continually presented to the sputtering zone which results in more uniform sputtering of the target material off of the tube. While rotating magnetron sputtering targets represent an improvement with respect to erosion, they can still experience uneven or non-uniform erosion of the sputtering material from the tube during sputtering—especially at the high sputtering rate areas proximate the target ends which are sometimes called turn-around areas/portions.
Unfortunately, the uneven sputtering of the target material off of the cathode tube can result in undesirable burn-through. Burning through the target material to the tube 20 would result in the sputtering of material making up the tube (e.g., stainless steel) thereby resulting in contamination of the sputtered film on the substrate. If allowed to continue, a hole could develop in the backing tube 20 which would allow cooling water from the tube interior to enter the sputtering chamber. Thus, it will be appreciated that burn-through to the tube 20 during sputtering represents a significant problem.
In view of the above, it will be appreciated that there exists a need in the art for a sputtering target constructed in a manner designed to reduce the likelihood of problematic burn-through.